Frequency converter for different mains voltages

ABSTRACT

The invention is directed to controlling an electric motor by means of a frequency converter. The frequency converter can be connected to one of several different mains voltages. According to the present invention, the maximum output power of the frequency converter, is limited when the actual mains voltage is lower than the maximum nominal mains voltage, for which the frequency converter is designed, and during the limitation the frequency converter controls the speed of the motor within a power range up to a limited maximum output power. This gives a genuine multi-voltage unit, which can be connected to a wide range of mains voltages.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of and incorporates byreference essential subject matter disclosed in InternationalApplication No. PCT/DK02/00785 filed on Nov. 21, 2002, Danish PatentApplication No. PA 2001 01750 filed on Nov. 23, 2001 and Danish PatentApplication No. PA 2001 01751 filed on Nov. 23, 2001.

FIELD OF THE INVENTION

The invention concerns a frequency converter, which can be connected toone of several different mains voltages, and a method for speed controlof an electric motor by means of a frequency converter.

BACKGROUND OF THE INVENTION

Differences in the mains voltages in the USA and Europe increase theproduction costs, as product differentiation is required. For years,therefore, it has been known to make frequency converters, which can beconnected to different mains voltages, see for example U.S. Pat. No.4,656,571.

However, this gives rise to a problem, when a frequency converter isconnected to a mains voltage, which is lower than the maximum nominalmains voltage, for which the frequency converter is designed to handle.In this case the current in the unit will be increased in order tomaintain the same electrical power output to the motor. The increasedcurrent may damage the frequency converter. To avoid this, the frequencyconverter can be designed for the high current, but this is an expensivesolution, which will also require more room, because, for example, theintermediary circuit coil gets bigger. Thus, protection of the frequencyconverter is required.

Reduction of the current flowing in a frequency converter is known fromEP 0 431 563 B1, however, in another connection. When the load, that is,the motor which is here used in a compressor for an HVAC system,consumes too much current, a control device reduces the output frequencyof the inverter. The amplitude of the current is detected by a currentmeasurement on the mains side, that is, before the frequency converter.When the output frequency is reduced, the current flowing to therectifier from the mains is reduced, and thus the installation inprivate homes is protected against over current. However, the frequencyconverter described is not intended for connection to one of severaldifferent mains voltages.

A mains voltage self-adapting frequency converter is described in theU.S. Pat. No. 4,656,571 mentioned above. In front of the rectifier avoltage detector is placed, which gives information to a control deviceabout the amplitude of the mains voltage. The control device comprises amemory element, in which a table for every possible mains voltage isstored. Each table again contains a number of U/f relations, known perse as the relation between the motor voltage and motor frequency appliedby the inverter. To obtain an optimum motor operation, this relationmust be kept constant. There are tables containing the U/f relations formains voltages of 100V, 115V, 200V and 230V, and the output power of thefrequency converter is regulated to a constant value by selecting thesuitable U/f relations. In this solution, it is endeavoured to keep theintermediary circuit voltage constant, and for this purpose the inputcircuit contains a switch arrangement, which switches to voltagedoubling mode when the unit is connected to 115V, whereas the switchingdevice remains unchanged when connecting the unit to 230V. Thus, themotor receives the same voltage, but this system has the disadvantagethat with a 115V mains voltage the input circuit has to sink a highercurrent. The complete input circuit has to be dimensioned for the lowestmains voltage, meaning that with the higher mains voltages thecomponents will be substantially overdimensioned. A further problem withthis design is that non-standard voltages are not considered. It is thusunclear, how the control device will handle a voltage supply drop from230V to 170V. A stepless operation is not described.

Self-adaptation of a frequency converter should not be limited toadapting to the mains voltages. Also, adaptation of current limits is ofinterest. Today, a frequency converter typically contains three currentlimits, the upper limit being a short-circuiting protection, which isactivated at a current of 300% of the nominal frequency convertercurrent, and has a response time of 1 to 2 μs. This limit is activatedwhen earthing or a short-circuit appears between the phase windings. Thesecond limit is the hardware limit, which is typically at a current of220% with a response time of 15 μs. The hardware limit is an expressionof the maximum current, which semiconductors and coils of the frequencyconverter can stand. The short-circuit limit and the hardware limit arerealised with electronic components, whereas the third limit, thesoftware limit, is controlled by the program of the frequency converter.A typical software limit is 160% current for 60 seconds, after which thefrequency converter reduces the load by lowering the motor frequency.

Normally, the hardware current limit is locked in the frequencyconverter already during manufacturing, but U.S. Pat. No. 4,525,660discloses an over current circuit, in which the current limit isvariable during operation. One input of a comparator here receives acurrent measuring signal from a current measured in the intermediarycircuit or on the motor lines, the other input receives a currentreference signal, which changes in dependence of the voltage-frequencyratio (U/f) of the frequency converter. When the current measuringsignal exceeds the current reference, the comparator sends a signal inorder to reduce the current or fully stop the frequency converter. Thevariable current reference signal consists of two contributions, a firstfixed contribution being set by means of a potentiometer and a secondcontribution being determined as a function of the U/f ratio. The secondcontribution is found by means of a lookup table or a functioncalculation, the U/f ratio being the entry key and the current limitcontribution increases with increasing U/f. Thus, the resulting currentreference signal changes during operation of the frequency converter.However, this circuit has the disadvantage that the variation of thecurrent limit is locked to the U/f ratio, which alone determines thecharacteristic, that is, the curve profile, of the current reference.

WO 97/36777 also describes a circuit for generating current references,which change during operation of a motor control, and the change takesplace as a function of the speed of a vehicle. A current referencegenerator generates a hardware limit value by means of a table in amemory element comprising combinations of speed and current, while amicroprocessor forms a software current limit. Via data lines, themicroprocessor is connected with the memory element of the currentreference generator, thus being able to change the profile of thehardware characteristic. Compared with U.S. Pat. No. 4,525,660, thecircuit has the advantage that the current limit characteristic can bechanged merely by changing the programming of the microprocessor. Thisgives the manufacturer a larger degree of freedom. Larger degrees offreedom are particularly desirable, when the manufacturer wants to makea general purpose motor drive, which can be used for different sizes ofmotors. A typical problem is the series-manufactured high-powerfrequency converter being connected to a low-power motor. The hardwarecurrent limit of the frequency converter is then locked and too high inrelation to the motor. A further degree of freedom is desirable withregard to application areas, that is, areas in which the frequencyconverter is used. For example, HVAC (Heating, Ventilation,AirConditioning) and conveyor belt applications have different currentlimit profiles, but usually they are fixed in the frequency converteralready during manufacturing. Thus, the frequency converter manufactureris forced to have many variants, each having its particular hardwarecurrent limit.

SUMMARY OF THE INVENTION

Based on the above, the object of the invention is to design a generalpurpose frequency converter, which electrically can adapt, or can easilybe adapted, to its surroundings.

According to the invention, this task is solved by a method where aDC/DC converter placed between a rectifier and an inverter is regulatedto output a DC voltage that is kept constant during operation at eachone of the several mains voltages, that a limit value signal is comparedwith one ore more measured or calculated parameters, that a controldevice limits the maximum output power by reducing the motor frequencygenerated by the inverter, and that the frequency converter, during alimitation of an output power of from the inverter, controls the speedof the electric motor within a power range up to the limited maximumoutput power.

This solution provides a universally applicable frequency converter,which can, in principle, be connected to a wide, continuous spectre ofmains supply voltages. The frequency converter adapts automatically tothe available mains voltage by limiting the output power in dependenceof the mains voltage to which the frequency converter is connected. Ifthe frequency converter e.g. is designed for a maximum nominal mainsvoltage of 230V, which will appear from the specifications of the motorcontroller, the control device will reduce the output power whenconnected to an actual mains voltage of 115V. More accurately, thecontrol device will limit the maximum output power, so that thefrequency converter is moved from one expected working point to another,after which the frequency converter will continue working in the newworking point. The invention is applicable both in the situation wherethe frequency converter is initially connected to a mains having a lowerinitial voltage and working for the rest of the operation period withthis voltage, and in the situation where a drop in the mains supplyvoltage occurs during an operation period. Throughout the duration ofthis lower voltage, the maximum output power is limited, until the mainssupply voltage reaches its nominal value again. In this way, the controldevice protects the frequency converter against a too high current, andthe result is a universally applicable motor controller. Even though themotor is derated, it will in many cases be more practical for both theOEM manufacturer and the end user only to have to handle one type offrequency converters and motors in his warehouse. Independently of thekind of limit value used, the output power of the inverter can bereduced by reducing the motor frequency. The motor frequency is thefrequency applied by the inverter, and is, in a manner known per se, indirect relation to the speed of the motor. As the power consumption fore.g. a pump or a fan motor is proportional with the speed in the 3^(rd)power, a reduction of the motor frequency will have a clear effect onthe power consumption and thus on the current consumed by the frequencyconverter. Instead of using a controlled rectifier, which is typicallyused with multi-phase mains supplies, this invention uses a boostconverter arranged after the rectifier, the output voltage of the boostconverter being controlled by the control device to be constant. Keepingthe DC voltage constant in the intermediate circuit during operation atone of the several mains voltages gives robustness towards suddenchanges in the mains voltages and provides a well defined designplatform with the possibility of using lower rated components due to theinventive power limitation. This especially applies when the DC voltagehas the same constant amplitude at different mains voltages. This givesthe advantage of the full speed range independent of the amplitude ofthe mains voltage. Another variant is changing the amplitude of the DCvoltage in dependence of the different mains voltages, but still keepingthe DC voltage constant during operation. The DC/DC converter,preferably a boost converter, offers in a known manner power factorcontrol and intermediary circuit voltage control in one.

The method according to the invention offers large advantages in areaswith socalled weak mains, that is, islands, where the amplitude of themains voltage is particularly sensitive towards the amperage drawn.Imagining that a frequency converter according to the state of the artis connected between a motor and a weak mains, and the weak mains has alower voltage than expected, this will cause an increased current sinkfrom the mains, which again results in a voltage drop, etc. Theinvention breaks this negative circle, as the reduction of the inverteroutput power prevents a further reduction of the mains voltage. Thus,the invention contributes to increased operational reliability.

In principle, the control device can reduce the output power by apredetermined fixed value, if the actual mains voltage is lower than themaximum nominal mains voltage, for which the frequency converter isdimensioned. The control device can then be informed of the amplitude ofthe mains voltage via a setting, which the user makes when commissioningthe unit. However, it is preferred that the control device performs anindependent and automatic adaptation of the frequency converter to themains voltage, and therefore the control device contains a limit value,which is compared with one or several measured or calculated motorcontrol parameters, for example the rectifier current. If the result ofthe comparison gives that the measured or calculated parameter is higherthan the limit value, the output power is reduced.

The reduction of the output power can take place shortly afterconnecting the frequency converter to the mains. If, at the end of thestarting period, the measured or calculated parameter is higher than thelimit value, the control device limits the maximum output power of thefrequency converter and, in principle, maintains this new valuethroughout the remaining operation period. With this immediatedetermination of the maximum permissible output power it is avoided thatthe control device has to use calculation power during operation.

Instead of using only one limit value, different limit values can bestored in a table, from which the control device selects one on thebasis of one or more measured or calculated parameters. For example, thesize of the limit value can vary with the temperature measured near thesemi-conductors.

A particularly accurate formulation of the limit value can be obtainedby making the control device calculate or determine the limit valuecontinously during operation. Even though this requires calculationpower, this is compensated by increased performance because thefrequency converter will work in its optimum working point at the mainsvoltage in question. In other words, the motor is controlled so that itsupplies the maximum acceptable power at a given mains voltage.

Advantageously, the limit value can be expressed as a current limit, forexample, by the current in a motor phase or—which is preferred—by thecurrent flowing after the rectifier. Due to the interrelation betweenmotor frequency and input current—higher frequency causes highercurrent—it is expedient to let the current limit be a function of themotor frequency, possibly combined with a measured temperature.

The reduction of the motor frequency is made by creating a frequencydifference between a desired motor frequency and a frequency-dampingterm, the frequency-damping term being calculated on the basis of thedifference between the measured or calculated parameter and the limitvalue. The frequency difference is then led on to the inverter as areference signal.

The control is mainly based on a U/f control, meaning that a reductionof the frequency f will also cause the motor voltage U to drop, becauseit is desired to keep the relation U/f constant to ensure correctmagnetisation.

With a lower motor voltage, it is possible to reduce the intermediarycircuit voltage. This reduction is done by the DC/DC converter, andafter this reduction the intermediary voltage will again be keptconstant during the operation. The advantage of the reduction of theintermediary circuit voltage is that less heat generating electricalpower is deposited in the intermediary circuit components, which resultsin a reduced thermal load.

The mains voltage can be measured directly by means of a voltage sensoron the input of the rectifier, but it is also possible to get anindirect expression of the amplitude of the mains voltage by arrangingcurrent sensors in the intermediary circuit of the frequency converter.

By inserting a measuring resistor between the rectifier and the boostconverter and a measuring resistor between the boost converter and theinverter two current values are obtained which can be compared to twolimit values. The first limit value is compared to the current in therectifier, and determines when the output power of the inverter shouldbe lowered. The second limit value is compared to the current in theinverter and relates to the over current protection of the inverter.Advantageously, the second limit value is made variable as a function ofthe nominal power rating of the motor, the actual motor frequency or thetorque load on the motor shaft.

The invention also relates to a frequency converter for speedcontrolling an electric motor. The frequency converter according to theinvention comprises a boost converter between rectifier and inverter,which boost converter generates a DC voltage which is kept constantduring operation at each one of the several mains voltages, and furthercomprises a control device which lowers the output power of thefrequency converter by means of a first limit value as already describedin order to adapt to the mains voltage, but additionally to the firstlimit value a second limit value is introduced which represents thehardware current of the inverter and which is adjustable duringoperation of the frequency converter. The second limit value is comparedto a current measuring signal and an over current signal is given oncethe current measuring signal exceeds the second limit value.

In this way a universally applicable frequency converter can bedesigned. On the mains supply side, the frequency converter adapts tothe available mains voltage by limiting the maximum output power,whereas on the motor side the frequency converter adapts to the motor byvarying the limit value of the over current signal in accordance withthe motor size, the motor frequency or the load connected with the motorshaft.

Preferably, the second limit value is generated on the basis of areference characteristic stored in a memory. By enabling the currentreference characteristic to depend on different variables, the desireddegree of freedom for the frequency converter manufacturer is obtained.The current reference characteristic is almost completely open formodifications, as one characteristic with a first variable can bereplaced by another characteristic with a second variable. One can haveone frequency converter, which is adapted to the motor size, or one canhave one power part comprising, among other things, the inverter, thesame power part being applicable for different sizes of frequencyconverters. The only part to be replaced is the control part, which isthen programmed with exactly the current limit applying for the powerpart in question. Even though the power part components will in manycases be overdimensioned in relation to the motor, for example a 250 Wfrequency converter with a connected motor of 150 W, it is cheaper forthe frequency converter manufacturer to have few variants. Large-scaleadvantages and flexibility are achieved, because the adaptation can takeplace at manufacturing, or at the OEM manufacturer or at the end-usersplace. The current reference characteristic is stored in a memoryelement, and the replaceability of the characteristic gives the desiredflexibility. Thus, in one variant one can program one characteristic,which is dependent on one or two variables, while in another variant onecan program another characteristic dependent on two other variables. Thecharacteristic can be formulated in dependence of many variables and beexpressed as any function in a formula or in a table. The characteristiccan e.g. be stored as a fixed characteristic in the microprocessorprogram—if desired, several selectable characteristics. The second limitvalue is pulse-width modulated and generated by the reference generator.This gives a simple and accurate control of the signal. This has thefurther advantage that, besides the short-circuit limit, the frequencyconverter can work with only one additional current limit, because thesoftware current limit and the hardware current limit have been combinedto one. This means a reduction of the required number of components.

These variables can be chosen from the control parameters as motorfrequency, U/f ratio and temperature, or from the application parametersas motor size or load torque characteristic. If for example thetemperature is chosen as input variable, the second limit value willdecrease with increasing temperature. Alternatively, the current limitcan be formulated as a function of both a control parameter and anapplication parameter.

The control and application parameters, or a mix of same, are sent asinput signal to the reference generator, which is made as an FPGA (FieldProgrammable Gate Array), ASIC (Application Specific IntegratedCircuit), DSP (Digital Signal Processor) or a microprocessor.

It is preferred that at least the motor frequency is used as a variablein the characteristic, as the motor frequency is a good expression ofthe current in the frequency converter and motor. Typically, it appliesthat increasing frequency gives increased current, and different currentlimits belong to different frequencies.

If the current reference characteristic is formulated with bothtemperature and motor frequency as variables, a particularly accurateexpression of the hardware current limit is achieved.

One problem is that the current generated with a blocked rotor isapproximately equal to the starting current of the motor. Therefore, theover current circuit can normally not distinguish between the twosituations, but varying the second limit value so that the level isincreased from the start up to a limit frequency and then is reducedagain, a possible wrong detection is avoided. For a short while, thehardware current limit is simply increased above the amplitude of thestarting current.

Before comparison, the pulse-width modulated signal should be convertedto an analog signal, and this is most easily done by means of a low-passfilter. The comparison of the measured current and the current referenceshould be made with analog technique to achieve the required speed.

The control and/or application parameters meant to form the basis of thecurrent reference characteristic will mainly be determined duringmanufacturing of the frequency converter, but can also be madeselectable by the user, for example, an OEM customer settingdip-switches or jumpers arranged in the frequency converter orprogramming the frequency converter by means of serial communication,for example by using a PLC.

Alternatively, the frequency converter itself can adjust the currentreference characteristic. The frequency converter measures the motor,determines its electrical parameters and calculates the motor size.Automatic parameter measurement is known from the state of the art andis usually made before starting the motor.

Advantageously, the current signal measured by the frequency converteris standardised with a standardising circuit, so that the control doesnot have to know the absolute value of the current. This means that thesame control can be used for different sizes of frequency converters.

In the type of frequency converters, in which the power part and thecontrol part are separated and placed on two different printed circuitboards, the standardising circuit can be arranged on either of thesecards. This again means that e.g. identical power parts can be used fordifferent control cards or identical control cards can be used fordifferent power cards, as, in a manner of speaking, all adaptationalfunctionality is placed on the control card. It is preferred that thestandardising circuit is arranged on the power part.

The standardising circuit can be made in a simple manner by a parallelconnection of two serially connected resistors with a current signal.Preferably, the current signal is generated by a measuring resistorarranged in the intermediary circuit of the frequency converter.

The object is also reached by means of a method for generating an overcurrent signal.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be explained on the basis of thedrawings, showing:

FIG. 1 shows the electronic circuit for a frequency converter with anelectric motor connected

FIG. 2 shows a first curve profile for the power limitation over time

FIG. 3 shows a second curve profile for the power limitation over time

FIG. 4 shows a first embodiment of a controller used in the invention

FIG. 5 shows a second embodiment of a controller used in the invention

FIG. 6 shows a third embodiment of a controller used in the invention

FIG. 7 shows a diagram of a circuit for the detection of over current

FIG. 8 shows a curve profile over time of an over current limit value

FIG. 9 shows a curve profile of an over current limit value as afunction of the motor frequency

FIG. 10 shows a circuit for controlling the Power Factor Corrector

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The frequency converter 1 has a rectifier 2 that is connected to asupply mains 3, which can have one of the standard voltages 115V, 230Vor any other value. In the following, “nominal voltage” means thespecified value, whereas the “actual voltage” means the real, availablevoltage. The frequency converter is built into a washing machine, andthe supply mains is here shown as one-phase mains, but in principle alsomore phases are possible. A three-phase motor 5 is connected to aninverter 4, and the inverter is controlled by a control device 6. Thecontrol device contains memory elements (RAM) and can be made as a microcontroller, DSP (Digital Signal Processor) or ASIC (Application SpecificIntegrated Circuit) with integrated or external memory. In a knownmanner the rectifier converts the alternating voltage of the supplymains to a direct voltage, which via the power semiconductors 7 of theinverter, for example IGBTs (Insulated Gate Bipolar Transistor), isreconverted to an alternating voltage on the motor lines 8. Atemperature sensor 24 is arranged near the power semiconductors. A boostconverter 9 comprising a coil 10, a switch 11, a diode 12 and acapacitor 13 is used to adjust the intermediary circuit voltage U_(dc)to a substantially constant value of 350V, so that it is possible toconnect the inverter to one of several mains voltages. Though a boostconverter is preferred, a buck converter or a sepic converter might alsobe used. The control device 6 measures the voltage before and after theboost converter via the signal lines 15 and 16, and measures the currentin the minus conductor before and after the boost converter via twomeasuring resistors 20 and 21 and the signal lines 22 and 23. The switch11 is connected to the control device via the signal line 25. At thesame time, the boost converter acts as a Power Factor Corrector (PFC),working in a manner known by making the current in the boost coil followthe curve shape of the rectified mains voltage. The power factor on themains supply side is very close to 1. Instead of using a boostconverter, it is also possible to use a controlled rectifier to obtainthe adjustment of the intermediary circuit current, but it is slightlymore difficult to obtain a high power factor. For a one-phase mainssupply, a controlled rectifier can consist of two or four thyristorswhich are connected to the control device.

The invention works as follows. Assuming a motor size of 750 W, a mainsvoltage U_(N) of 230V will result in a mains current I_(N) of about 4.8A as input current for the rectifier. If instead the mains voltage were115V, the same motor load would cause an input current of about 9.6 A tofulfil the power demand of the motor. Unless the rectifier 2 and thecoil 10 have been dimensioned accordingly, they will either be damaged,or the coil will generate a very high loss heat, which can damage theremaining electronics. Further, the core of the coil may be saturated,which results in loss of self-induction. This will cause high peakcurrents, which will result in further increased losses, which maydamage the semiconductors. Thus the cooling is a large problem, when afrequency converter is arranged in a closed chamber in a washingmachine, and this problem is aggravated, when the mains voltage is low.In a possible first embodiment the control device adapts the outputpower of the frequency converter by comparing a measured current I_(rec)after the rectifier with a current limit value I_(lim) which applies fora mains voltage of 230V. The control device adjusts the output power ofthe frequency converter so that the current I_(N) of 4.8 A is notexceeded, thus limiting the maximum power output of the frequencyconverter to 375 W. FIG. 2 shows this situation. At the time t0, thefrequency converter starts with a limit P_(max1) for the maximumelectrical output of 750 W. In the period from t0 to t1, which isslightly longer than the starting time, and lasts about 3 seconds, themains voltage or the intermediary circuit voltage is detected andcompared with a limit value stored in a table. If the voltage is lowerthan the limit value, the control device concludes that the mainsvoltage supply is lower than the value for which the frequency converteris dimensioned, and limits its maximum output power to P_(max2) of 375W. This value is kept constant for the rest of the operation time,during which the frequency converter under the limitation controls thespeed of the electric motor within the power range up to the limitedmaximum output power P_(max2).

As mentioned above, a direct measurement of the amplitude of the mainsvoltage is not required, as the amplitude of the mains voltage can becalculated by the control device on the basis of the current in therectifier and, for example, the voltage in the intermediary circuit. Noris it necessary to know the exact amplitude of the mains voltage, as anindication of the amplitude of the mains voltage is sufficient, and thisindication may for example consist of a current measurement right afterthe rectifier via the measuring resistor 20. The instant of a limitationof the output power may be shortly after the start of the frequencyconverter, for example when the boost switch 11 has started to switchand the charging of the capacitor 13 is finished and an indication ofthe amplitude of the mains voltage has been obtained. At this time, thecontrol device can set a suitable limit value, which applies for therest of the operation period of the frequency converter. This solutionsaves calculation power. Or, as previously mentioned, the control devicecan make a continuous surveillance during operation, and if the limitvalue is exceeded then reduce the electrical output power.

The power limiting mechanism is also activated if the mains voltagesupply fluctuates. Typically, a frequency converter will be dimensionedfor the nominal mains voltage plus/minus a fluctuation range, forexample 230V+/−10%. However, when particularly weak supply mains areconcerned, voltages down to e.g. 180V may occur, and here the inputcurrent will increase to meet the power requirements of the motor.According to the invention, however, the control device will reduce themagnitude of the maximum output power to a value which is acceptable tothe frequency converter. Assuming that a motor of 1500 W is connected, anominal mains voltage of 230V will cause a current of 9.5 A in the inputcircuit. FIG. 3 shows the maximum power limit P_(max1) during the periodt0 to t1. At the time t1, the mains supply drops to 180V, and thecontrol device limits the output power to P_(max2) of 1200 W because thelimit value has been set to 9.5 A. The throttling of the maximum outputpower takes place during the period t1 to t2. The performance of themotor is derated, but the motor is still working rather than beingstopped by the control device in order to protect the frequencyconverter. This corresponds to a reduction of the maximum centrifugingspeed in the washing machine. Often the end user does not care whetherthe centrifuging takes place at 1100 RPM or 800 RPM. Other applicationscan also be mentioned, e.g. a pumping system, in which the pump runs ata lower speed in order that at least some water can be pumped, or arefrigeration unit working at a reduced capacity rather than stoppingcompletely. Back in FIG. 3 it happens at the time t2 that the mainsvoltage supply increases slightly, namely to 200V. The control devicenow increases the limit of the maximum power limit P_(max3) to 1320 W.This is less than what the motor can stand, but still a higher powerlimit than during the period t1 to t2.

FIG. 4 shows in a block diagram how the control algorithm, which reducesthe output power, can be made. The controller shown in FIG. 4 is part ofthe control device 6, and the output power is reduced by means of areduction of the motor frequency based on a measurement of the rectifiercurrent. It is assumed that the intermediary circuit voltage U_(dc) isconstant. The measured rectifier current I_(rec) is averaged in a filter30 and subtracted from the current limit value I_(lim) in a subtractor31. In a converting unit 32, the current deviation is converted to afrequency f_(lim), which is a frequency reduction term and expresses themagnitude, with which the motor frequency must be limited, for example10 Hz. The converting unit 32 contains a mathematical transfer function,but it can also be made as a controller. In a frequency-converting unit33, which contains a mathematical transfer function, the frequencylimiting term f_(lim) and the desired motor frequency f_(set) areconverted to an output signal f_(ref). In the simplest embodiment theconverting unit 33 is merely a subtractor but preferably it is made witha filter function for controlling the frequency change per time unit.The output signal from 33 is the reference frequency f_(ref), which isled direct to a pulse width modulating unit 35, which controls thesemiconductors in the inverter 4. Further, f_(ref) is sent into a U/fcontrol unit 34 which on the basis of this signal and the measuredintermediary circuit voltage U_(dc) generates a signal for themodulation index m_(a), that is, the relation between the reference forthe motor voltage and the intermediary circuit voltage U_(dc).Alternatively, the duty cycle D can be used, i.e. the relation betweenon-time and period time of the semiconductors, but there exists a dutycycle for each motor winding i.e. three in total, which makes a controlwith D difficult. Further, the switching frequency of the inverter iskept constant.

Another control algorithm for limiting the output power of the frequencyconverter at low supply voltages is shown in FIG. 5. Also here it isassumed that the intermediary circuit voltage is constant. The voltageU_(rec) after the rectifier is measured and smoothed in a filter 40. Thesignal is then led into a unit 41 which converts the voltage to a powerlimit value signal P_(lim). The unit 41 contains a table of differentsizes of the limit value P_(lim), in which the voltage U_(rec) is usedas entry. This reference value is led to a subtractor 42, and the actualpower output P_(inv) from the inverter is deducted from the limit value.The actual power output is calculated in the unit 43 on the basis offiltered values of a measured intermediary circuit current I_(dc).Possibly, the power calculation can also comprise a measuredintermediary circuit voltage U_(dc). This is shown by means of dashedlines in box 44 which contains a filter. The difference between P_(lim)and P_(inv) is then led into a converting unit 45, which converts thedeviation to a frequency f_(lim), which expresses the frequencymagnitude, with which the motor frequency must be limited. Theconverting unit 45 is made as a regulator or as a transfer function. Theremaining circuit is identical with that in FIG. 4. The power P_(inv)could also be calculated on the basis of motor voltage and motorcurrent.

While the regulators in FIGS. 4 and 5 work on the basis of a fixedintermediary circuit voltage U_(dc) which is generated by the boostconverter, the control circuit in FIG. 5 is based on a variableintermediary circuit voltage. The intermediary circuit voltage U_(dc) isbrought to vary concurrently with the supply voltage by controlling theboost switch 11 in accordance with the fluctuations of the supplyvoltage. Thus, the voltage after the boost converter is used as anindirect expression of the amplitude of the mains supply. The advantageof using this type of intermediary circuit instead of the intermediarycircuit with constant voltage is that the intermediary circuit voltageonly has to be boosted to the required value, and not higher. The largerthe distance between the RMS-value of the mains voltage and the voltagein the intermediary circuit, the larger the losses. The fluctuations ofthe supply voltage are detected by measuring the voltage U_(rec). Thevariable voltage U_(dc) is led into a converting unit 50 which convertsthe voltage to a frequency f_(lim), which expresses the magnitude withwhich the motor frequency must be limited. After that, the circuit worksas described in FIGS. 4 and 5. Alternatively, the frequency limitationf_(lim) can be determined as a function of the modulation index via aconverting unit 51. To show the option, the boxes are drawn with dashedlines. If the mains supply voltage drops, thus pulling down theintermediary circuit voltage U_(dc), it is no longer possible to supplythe motor with the correct voltage, but by reducing the motor frequencyas a function of either the modulation index or the intermediary circuitvoltage, it is achieved that the motor can be supplied with the correctvoltage, and at the same time the output power is reduced as a functionof the supply voltage.

We now revert to FIG. 4 and the limit value I_(lim). This limit valuecan be made dynamic and dependent on different parameters, for examplethe temperature T measured by the temperature sensor 24 (FIG. 1), and bestored as fixed values in a reference table or be continuouslycalculated by the control device during operation.

FIG. 4 also shows a signal OC, which is led into the pulse widthmodulating unit 35. This signal is an over current or a trip signalwhich turns off the inverter if the absolute limit for what theelectronics can stand is reached. In the state of the art, the hardwarecurrent limit signal OC is most frequently generated when a fixedcurrent limit value is exceeded, but advantageously, the current limitfor tripping can be made variable and dependent on the motor frequency.FIG. 7 shows a circuit 60 which generates the over current signal OC.The circuit components 61, 63 and 70 are parts of the control device 6.A reference generator 61 generates the variable limit value on the basisof one or more variables, namely the parameters A1, A2, AN or S1, S2,Sn. “A” means application specific parameters, such as the quadratictorque load on the motor shaft of a pump, whereas “S” means controlparameters such as motor frequency, motor temperature and invertertemperature. These have the reference number 62. On the output 63, thereference generator supplies a pulse width modulated signal with asuitable frequency, for example 2 kHz. The duty cycle is set at 50%,which results in a current limit reduced by 50%. In a low-pass filter64, the PWM signal is converted to a direct voltage signal I_(OC) on theinput 65 to the comparator 66. The signal on the input 65 represents thevariable limit value. On the input 67, there is a signal for the actualcurrent measurement, for example a signal representing the magnitude ofthe intermediary circuit current. The output signal from the comparator66 is led via the connection 71 to the reference generator, which is inthe form of an FPGA or a microprocessor. Even though digital technology,which is slower than the corresponding analog technology, is used forgenerating the hardware current limit, an acceptable response time isachieved as the digital processing is limited to the change of thecurrent reference signal, that is, the signal on the output 63. Thecircuit is sufficiently fast because generation of the over currentsignal takes place with analog circuitry on the output 69. A circuit 68consisting of ohmic resistors ensures that the current signal is alwaysstandardised—in the present case, 220 mV on input 67 corresponds to themaximum permissible current in the inverter. If the signal is instead150 mV it corresponds to an actual current utilisation of the power partof the frequency converter of 68.2%. In this way, the control devicedoes not need to know the power size of the power part in the frequencyconverter. As shown in FIG. 7, the standardising circuit has been madeby means of a parallel connection of two shunt resistors with themeasuring resistor in the minus conductor of the intermediary circuit.The size of the measuring resistor is 18 milliohms, but the sizes of theshunt resistors differ. The shunt resistors work as voltage dividers andare in the range of a few megaohms. The standardising circuit 68 ispreferably arranged on the printed circuit board of the power part andforms part of the interface to the control bord when an electricalconnection of power part and control part is required.

To the manufacturer of frequency converters the standardising circuitmeans that the same printed circuit board containing the control partcan be used for power parts of different sizes. When the direct voltagesignal on input 65 exceeds the current measuring signal on input 67, thecomparator output OC on line 69 goes high and indicates “over current”,after which the control device 6 decides, whether or not to turn off thefrequency converter. The variable current limit I_(OC) is primarilyexpressed as a function of the actual motor frequency f_(ref) andpossibly one or more of the electrical parameters of the motor. Theseparameters—A1, A2, An and S1, S2, Sn in FIG. 5—can be the electricalpower of the motor or the nominal current of the motor, which can beread direct from the motor label. Alternatively, motor parameters likestator resistance, rotor resistance and inductances, such as main fieldinductance and leakage inductance, can be part of the determination ofthe current reference characteristic. On the basis of these parametersthe reference generator 61 can directly read or calculate the size ofthe motor which is connected to the frequency converter, and thusdetermine the limit value to be applied. In other words, the referencecurrent generator 61 adapts the current limit loc to the motor, so thatdifferent motor sizes can be connected to the same frequency converter.This particularly applies when a motor with a smaller power rating isconnected to a frequency converter with higher power rating. Thus, theuser does not have to readjust the current limit; this is doneautomatically by the frequency converter. The information about themotor size can be entered by the user via either a keyboard, a serialcommunication connection, jumpers or dip-switches arranged in thefactory on the control board, each dip-switch stating a motor size.However, it is particularly preferred that the control device by meansof the frequency converter measures the electrical parameters of themotor and uses this information to create the limit value I_(OC).Measuring the electrical parameters of the motor before operation isknown, and is made by injection of DC and AC signals in the statorwindings.

The variable current limit on input 65 in FIG. 7 can also be set by thereference generator on the basis of the typical load curve of theapplication. A memory 70 contains a table of related current limitvalues and the load curve, and e.g. in a pump application the curve willbe parabolic. In another example, in which the current referencecharacteristic is a sole function of the inverter temperature which ismeasured by the sensor 24 (FIG. 1), the course of the curve is anegative parabola having its peak at 0° C. and decreasing at increasingtemperature. In the example described, the current reference can e.g.follow this characteristic:I _(OC)=−0.0085·T ²+100  (1)where T is the temperature and I_(OC) is the current reference signal.In practice, the curve will be made piecemeal linear so that I_(OC) hasa fixed, constant value up to about 90° C., after which the curve willfall concurrently with the increasing temperature. The transfer function(1) can be realised as a function in the microprocessor or as a lookuptable.

As previously mentioned, the current reference signal can also be madedependent on the motor frequency or the reference signal of the motorfrequency (f_(ref) in FIG. 4). The characteristic will then be as shownin FIG. 9, where the current limit is increased during the start of themotor until a limit frequency fg is reached.

Further, the reference generator can vary the current limit during thestart of the motor as a function of time. FIG. 8 shows a characteristiccourse of the current limit I_(OC) from the start until a certain timeof the operation has lapsed. Via a dipswitch on the control board thecurrent limit has been set at I_(OC1) which matches a certain motorsize. In reality, the frequency converter is able to supply a muchlarger current, but the power rating of the motor is smaller. In theperiod t0-t1, the reference generator 61 increases the limit value toI_(OC2) to permit a larger starting current to pass without causingtripping. At the time t1, the reference generator reduces the currentlimit to I_(OC1) again. At the time t2, the ambient temperatureincreases, which causes a higher heat load of the electronicscomponents, and therefore the current limit is further reduced toI_(OC3). The current limit could also be reduced because of a drop inthe mains voltage. The reduction during the period t2 to t3 can beeither stepwise or continuous. One advantage of using the variablecurrent limit is that the limit value I_(OC) does not have to bedimensioned in accordance with the starting current, and this enables amore accurate detection of occurring malfunctions. The over currentgenerated by a blocked rotor can have approximately the amplitude of thestarting current, and in case of a fixed current limit of approximatelythe starting current level this over current will not be interpreted asa malfunction.

Alternatively, the current reference characteristic can be formulatedwith both the temperature and the motor frequency as variables.

As seen in the discussion concerning FIG. 7, splitting up the electroniccircuit in a digital part dealing with not time critical functions andin an analogue part performing the fast operation gave the advantage ofdigital flexibility and analogue speed. In FIG. 10 this principle iselaborated further with a circuit used for controlling the boostconverter 9. The boost converter performs as mentioned earlier a PFCfunction. A PFC control circuit consists of a (slow) voltage controlloop, a multiplier, multiplying the output of the voltage control loopwith the line voltage waveshape and a (fast) current control loop. Theoutput of the multiplier is reference for the current control loop. Thecurrent control loop could be an average current control, or a peakcurrent control. Such PFC control can be made as analogue circuit, aspure digital control (e.g. with a DSP), or as a mix of digital controland analogue circuitry

Even when using a split digital-analogue configuration of the type shownin FIG. 7 for controlling the boost converter, i.e. using amicroprocessor for generating a digital reference signal which islowpass filtered into an analogue reference signal, the PFC controltakes up much of the capacity of the microprocessor. Further, theresolution of the reference current waveshape generated by themicroprocessor is poor, because the PWM output is only updated every,say, 1 millisecond. The 1 millisecond also causes an unwanted phaseshift in the current.

This problem can be alleviated by moving the multiplier outside themicroprocessor and realize it as an analogue multiplier.

This will reduce the requirement for high update rate, and thereby freeup capacity in the microprocessor. Also, improvement of the waveshaperesolution and reduction of the phase shift will take place. The analogmultiplication can be made by PWM modulating the line waveshape signal.

The rectified voltage U_(REC) in FIG. 10 is divided down to signal levelby resistors 82 and 83. A PWM signal from the microprocessor 61′ is viaconnection 88 fed through resistors 80 and 81, and gates the dividedU_(REC) signal on and off by means of transistor 85 and diode 84. Diode84 ensures essentially the same impedance (approx. resistor 82) whenseen from capacitor 92 in on and off state. In the off state, diode 84compensates the voltage drop in transistor 85. In the on state, thevoltage drop of diode 84 is in series with the full U_(REC) voltage andtherefore has no significance. Resistor 82 and capacitor 92 filters outthe low frequency component of the signal on line 90. This signal is theproduct of the voltage U_(REC) and the dutycycle given by microprocessor61′, and the signal can be visualized as a series of digital pulseshaving constant width but varying amplitudes. However, capacitor 92 andresistor 82 smooth the signal into a varying analogue DC-signal calledI_(ref) in FIG. 10. I_(ref) represents that reference current to theboost converter 87, which gives in-phase current and voltage on themains side and is compared to a signal measured with resistor 95 placedin one leg of the boost switch. The output of the comparator is sent toa signal conditioning means 93, which is again connected to the gate ofthe boost switch. Signal conditioning means 93 contains an RS-flip flopand an amplifier. Output of comparator 87 resets the flip flop, whereasthe PWM signal from connection 88 sets the flip flop. The box 86 havingdashed lines represents the muliplicator function that was earlier afunctional part of microprocessor 61′ but is now made with fast discreteanalogue components. The circuit shown in FIG. 10 is used as a peakcurrent controller, but could be converted into an average currentcontroller.

Having in the above dealt with the generation of the over current signalOC on the basis of a variable current limit value, we now revert to alimitation of the output power in case of low mains voltage. Note thatthe current limit value I_(lim) and the current reference I_(OC) workindependently of each other as I_(lim) is an internal software currentlimit, while I_(OC) works as hardware current limit. The main functionof I_(OC) is to protect the semiconductors, and the signal OC cantherefore also be transferred to the boost switch 11.

As previously described, the limitation of the output power is effectedthrough a reduction of the motor frequency by the control device 6,which is done—via the connection 14—by changing the pulse-pause ratio ofthe power semiconductors in the inverter 4. As a U/f control is made,also the motor voltage is reduced. This means that at the same time theintermediary circuit voltage U_(dc) can be lowered to avoid anunnecessary boost of the intermediary circuit voltage. The limitationcan thus be made by means of a combination of the frequency reductionand the intermediary circuit voltage reduction, as the motor demand formotor voltage falls concurrently with dropping motor frequency. Theintermediary circuit voltage can be reduced by reducing the averagepulse-pause ratio on the boost switch 11 as a function of the outputpower of the frequency converter.

A reduction of the maximum limit of the output power protects the inputof the frequency converter against too high current. The speed of themotor is controlled in accordance with the mains voltage available onthe input of the frequency converter. Thus, the motor can not yield thesame torque as when connected to a higher mains voltage, but thisreduced performance is of course taken into consideration by theOEM-manufacturer when dimensioning the system.

1. A method for controlling an electric motor by means of a frequencyconverter, the method comprising the steps of: comparing a limit valuesignal with one or more measured or calculated parameters; and limitinga maximum output power when the one or more measured or calculatedparameters exceeds the limit value signal by reducing a motor frequencygenerated by an inverter; wherein the frequency converter is connectableto one of several mains voltages and a control device limits the maximumelectrical output power of the frequency converter when an actual mainsvoltage is lower than a maximum nominal mains voltage for the frequencyconverter, a converter between a rectifier and the inverter is regulatedto output an intermediary circuit voltage that is kept constant duringoperation at each one of the several mains voltages, and during saidlimitation of the maximum output power the frequency converter controlsa speed of the electric motor within a power range up to the limitedmaximum output power.
 2. The method according to claim 1, wherein on thebasis of the one or more measured or calculated parameters the controldevice limits the maximum electrical output power immediately afterconnection of the frequency converter to a mains voltage supply, if theone or more measured or calculated parameters exceed the limit valuesignal.
 3. The method according to claim 2, wherein the control devicecontains a table with limit values and that on the basis of the one ormore measured or calculated parameters the control device selects alimit value signal of a desired amplitude.
 4. The method according toclaim 1, wherein the control device determines the limit valuecontinuously during operation of the frequency converter.
 5. The methodaccording to claim 3, wherein the limit value signal is a current limitwhich is determined as a function of the motor frequency or a measuredtemperature, or a combination of the motor frequency and the measuredtemperature.
 6. The method according to claim 1, wherein a referencesignal for the motor frequency is generated as a difference between adesired motor frequency and a frequency reduction term, the frequencyreduction term being generated on the basis of a current difference, apower difference or the intermediary circuit voltage.
 7. The methodaccording to claim 1, wherein the control device reduces both motorfrequency and motor voltage.
 8. The method according to claim 7, whereinthe intermediary circuit voltage after the converter is also reduced. 9.The method according to claim 1, wherein the frequency convertercontains a voltage or current sensor for direct or indirect detection ofthe actual mains voltage.
 10. The method according to claim 1, wherein afirst current between rectifier and converter is measured and comparedto the limit value signal, and that a second current between converterand inverter is measured and compared to a second limit value, and thatan over current signal is given when the second current exceeds thesecond limit value.
 11. The method according to claim 10, wherein thesecond limit value is variable, and that the characteristic of thesecond limit value can be determined as a function of the nominal powerrating of the motor, the actual motor frequency or the torque load onthe motor shaft.
 12. A frequency converter for controlling an electricmotor, the frequency converter comprising: a rectifier for providing arectifier output voltage; a converter for regulating the rectifieroutput voltage to produce an intermediary voltage; an inverter forinverting the intermediary voltage and generating a motor frequency; anda control device; wherein said frequency converter is connectable to oneof several mains voltages, the intermediary voltage is kept constant ateach one of several mains voltages and the control device limits themaximum electrical output power of the frequency converter when anactual mains voltage is lower than a maximum nominal mains voltage forthe frequency converter by comparing a first limit value signal with oneor more measured or calculated parameters and reducing the motorfrequency generated by the inverter if the measured or calculatedparameter is greater than the first limit value signal; and wherein asecond limit value representing the hardware current limit of theinverter is adjustable during operation of the frequency converter, andthat an over current signal is given if a current measuring signalexceeds the second limit value.
 13. The frequency converter according toclaim 12, further comprising an electronic circuit for detecting an overcurrent condition, the electronic circuit including: a referencegenerator for generating the second limit value; a comparator whichcompares the current measuring signal with the second limit value whichis generated by the reference generator, the comparator giving the overcurrent signal once the current measuring signal is larger than thesecond limit value; and a current reference characteristic which isstored in a programmable memory element, the characteristic beingdependent on at least one first variable and replaceable by a secondcharacteristic which is dependent on at least one second variable;wherein the reference generator generates the second limit value as apulse-width modulated signal on the basis of the current referencecharacteristic loaded from the programmable memory element.
 14. Thefrequency converter according to claim 13, wherein the first and thesecond variables are chosen from a set of control parameters includingmotor frequency, motor temperature and inverter temperature, a set ofapplication parameters including motor size and torque loadcharacteristic of the motor shaft, or from a combination of a controlparameter and an application parameter.
 15. The frequency converteraccording to claim 14, wherein one or more variables of the type controlparameters and/or one or more variables of the type applicationparameters are sent to the reference generator as input signal.
 16. Thefrequency converter according to claim 15, wherein the motor frequencyis the first variable in the current reference characteristic.
 17. Thefrequency converter according to claim 14, wherein a parameter pairmotor frequency and inverter temperature is the first variable in thecharacteristic.
 18. The frequency converter according to claim 12,wherein the second limit value is held constantly at a first level frommotor start up to a limit frequency, after which the second limit valueis reduced.
 19. The frequency converter according to claim 13, whereinthe pulse-width modulated current reference signal is led into alow-pass filter, whose output is connected with an input on thecomparator.
 20. The frequency converter according to claim 14, whereinthe current reference characteristic is selectable by using dip-switchesor jumpers arranged in the frequency converter or by using serialcommunication.
 21. The frequency converter according to claim 14,wherein the frequency converter measures electrical parameters in themotor and determines the current reference characteristic on the basisof these parameters.
 22. The frequency converter according claim 12,wherein the second limit value is formed by a measured motor current ora measured current in the converter, and the measured current isstandardised in a standardising circuit having an output, the currentmeasuring signal being available on the standardising circuit output.23. The frequency converter according to claim 22, wherein thestandardising circuit is arranged on one of a printed circuit boardcomprising control electronics of the frequency converter and a printedcircuit board comprising power electronics, the two printed circuitboards being electrically connected with each other.
 24. The frequencyconverter according to claim 23, wherein the standardising circuitcomprises ohmic resistors, which ohmic resistors are connected inparallel with a current signal.
 25. A frequency converter forcontrolling an electrical motor, the converter comprising electroniccontrol circuitry wherein a digital controller generates a PWM signalwhich is led to an analogue lowpass filter and converted into ananalogue reference value which is a reference for a current controlloop, and an analog current measurement value is a feedback value forsaid current control loop; wherein the analogue reference value is ledto a first input of a comparator and the analogue current measurementvalue is led to a second input of the comparator; and wherein the PWMsignal is led into an analogue multiplier placed between the digitalcontroller and the analogue lowpass filter, said analogue multipliermultiplying the PWM signal to a signal representing a curve profile of amains voltage.